Rotational encoders or position sensors are devices that determine the angular position of a rotating target based on eddy currents induced into the target. An excitation coil induces the eddy currents into the target wherein the strength of the eddy currents are proportional to the position of the target relative to the coil. The eddy currents change the inductance of the coil, wherein the strengths of the eddy currents are proportional to the change in inductance in the coil. The inductance of the coil is measured to determine the angular position of the target. Position sensing of the target can be achieved in part by the shape of the target in the X-Y plane, by the height of the target along the z-axis, which is normal to the X-Y plane, or with both shape and height parameters.
One problem with the above-described position sensors is that the target can act as a large parasitic inductor if it is shaped such that conductive portions of the target form a closed current path. This large parasitic inductor causes an alternate method of coupling than was intended in the original target design. While the intended mechanism for measurement is in the generation of eddy currents in close proximity to the coil, the large parasitic inductor can cause additional eddy currents to be generated at much larger distances. These additional eddy currents decrease the total change in inductance in the excitation coil at close proximity, which reduces the accuracy of the position sensing.
Additionally, for systems with two or more excitation coils, the closed current paths resulting from the parasitic inductance in the target can also cause alternative paths of coupling between the coils at large distances. The closed current paths can be in either the lateral or vertical dimension and further diminish the accuracy of the position sensing.